Phase 2--Evaluation of High-Temperature
Asphalt Binder TestsUsing the RSCH and French PRT Mixture Tests at 70°C

A. Background

The polymer-modified asphalt binders used in this study had continuous high-temperature
PG's ranging from 71 to 77. The mixtures were tested by the French PRT at 70°C
. This was the highest temperature that can be applied by this tester. The mixtures
were tested for cumulative permanent shear strain at 50°C because of testing problems
encountered at higher temperatures in previous studies. However, most of the
problems were provided by the Frequency Sweep at Constant Height (FSCH) mode
of loading. RSCH and FSCH are both applied by the Superpave Shear Tester. It
was recommended that the diabase mixtures be retested using RSCH at a temperature
closer to the PG's of the asphalt binders.

B. Objective

The objective of phase 2 was to retest the diabase mixtures at 70°C using
RSCH to determine which asphalt binders provide high-temperature properties
that do not agree with mixture rutting resistance.

C. Materials

All 11 asphalt binders were used. Information on the asphalt binders, aggregates,
and mixture design are given elsewhere.(1-2) The reduced asphalt
binder content of 4.55 percent by mixture mass was used so that the mixture
met the 4.0-percent design air-void level as recommended by Superpave. Prior
problems with testing mixtures at temperatures above 50°C contributed to the
decision to lower the asphalt binder content. However, it confounded the analysis
of the data because the mixtures tested by the French PRT had a 4.85-percent
asphalt binder content.

D. Tests

High-temperature asphalt binder properties were measured by a DSR after RTFO
aging.(3) Mixture rutting resistance was based on the cumulative
permanent shear strains from RSCH and the rut depths from the French PRT.(4-5) All mixtures were subjected to 2 h of STOA at 135°C . Specimens were tested
48 h after compaction.

E. Cumulative Permanent Shear Strain

Cumulative permanent shear strain from RSCH was measured at 7.0-percent air
voids, 70°C , and 5,000 cycles. The applied shear stress was 69 ±5 kPa. The
loading time was 0.1 s and the rest time was 0.6 s. A minimum of three replicate
specimens were tested per mixture. Lower cumulative permanent shear strains
indicate more resistance to rutting.

The data at both 50°C and 70°C are given in table 6 and figure 4. The two
regression lines suggest that mixtures containing asphalt binders with the highest
PG's tended to be highly resistant to rutting at both test temperatures.
While this seems reasonable, the difference in asphalt binder content probably
decreased the vertical differences between the two lines.

Table 6 shows that the rankings are not the same at 50°C and 70°C . The only
certain difference in ranking is that the mixture with PG 70-22 performed relatively
worse at 70°C compared to 50°C . Figure 5 shows that the data point for
this mixture was furthest from the regression line. Being above the line, this
mixture performed worse than expected at 70°C , or better than expected at 50°C
. Because of the change in asphalt binder content, no firm conclusions could
be made for the other asphalt binders.

The cumulative permanent shear strains in table 6 indicate that grafting did
not improve the rutting resistance of EVA at either temperature at a 5 percent
level of significance. Grafting and geometry had no significant effect on the
rutting resistance of SBS at 50°C . All three SBS mixtures fell into group D.
SBS Radial Grafted did perform significantly better than SBS Linear at 70°C
. The shear strain for SBS Linear Grafted at 70°C was not significantly different
from the shear strains for SBS Radial Grafted or SBS Linear at 70°C .

The coefficients of variation (CV) for cumulative permanent shear strain at
5,000 cycles ranged from 1.7 percent to 36.7 percent. (See table 7.) Several
data points were found to be outliers. These outliers were not used when evaluating
asphalt binder properties. However, all of the replicate data are included in
table 7 because the variability of the data in table 7 is a good representation
of the variability typically provided by RSCH. The averages and CV's in the
parentheses include the outliers. At 50°C , the CV's ranged from 8.2 to 24.1.(1) Variability appeared to be greater at 70°C , but a paired t-test indicated that
it was not greater.

For the tests at 50°C , the cumulative permanent shear strains were correlated
to the G*/sind 's of the asphalt binders at three DSR frequencies: 10.0,
2.0, and 0.125 rad/s.(1) The G*/sind 's are given in table
8. These three frequencies provided r2's of 0.06, 0.55, and
0.89, respectively, using log-log transformations. The correlation depended
on DSR frequency. The correlation to high-temperature PG was 0.68 without transformation.
(Note: A log-log transformation was used if it provided a higher r2.
Relationships between asphalt binder and mixture high-temperature properties
are normally curvilinear. Thus, a log-log or power law transformation usually
provides a higher r2.)

Table 8. G*/sind 's of the asphalt
binders at 10.0, 2.0, and 0.125 rad/s with the
asphalt binders listed from highest to lowest G*/sind based on 0.125 rad/s.

Asphalt Binder

G*/sind at 50°C After RTFO Aging (Pa)

10.0 rad/s

2.0
rad/s

0.125
rad/s

EVA

26 300

12
100

2
740

EVA Grafted

35 800

14
300

2
310

Elvaloy

28 700

10
000

1
600

CMCRA

44 300

13
900

1
540

Air-Blown

49 100

14
200

1
390

SBS Linear
Grafted

25 600

8 000

920

ESI

32 300

8 900

870

SBS Linear

25 400

7 700

810

PG 70-22

40 700

10
200

810

SBS Radial
Grafted

25 100

7 600

800

PG 64-28

22 200

5 400

400

Table 6 gives the G*/sind 's at 70°C . When correlated against cumulative permanent
shear strain, frequencies of 10.0, 2.0, and 0.125 rad/s provided r2's
of 0.22, 0.37, and 0.59, respectively, using log-log transformations. This
correlation also depended on DSR frequency. The correlation to high-temperature
PG was 0.63 without transformation. All of the r2's are given in
table 9.

The correlations to G*/sind at 50°C and 70°C using a DSR frequency of 0.125 rad/s
are shown in figures 6 and 7, respectively. The correlation at 50°C is very good.
The largest deviation was provided by Elvaloy, followed by SBS Radial Grafted.
Figure 7 shows that the G*/sind 's for EVA and SBS Radial Grafted are low
at 70°C . G*/sind underpredicted their resistances to rutting. Because the data
for EVA significantly affected the position of the trend line, the trend line
in figure 7 was drawn without the data for EVA. The data point for SBS Radial
Grafted did not significantly affect the position of the trend line.

Figures 8 and 9 show the data at 70°C using DSR frequencies of 2.0 and
10.0 rad/s, respectively. Both figures show that the G*/sind for PG 70-22
is high, while the G*/sind 's for EVA and Elvaloy are low. The G*/sind for SBS Radial Grafted is low at 2.0 rad/s, but not at 10.0 rad/s based
on 95-percent confidence bands.

Figure 10 shows that the correlation using high-temperature PG is poor,
although the r2 increased from 0.63 to 0.79 after excluding the data
for SBS Radial Grafted. The 95-percent confidence band for cumulative permanent
shear strain at the mean PG of 74 is 22 000 to 36 000 mm/m with SBS Radial
Grafted and 25 000 to 36 000 mm/m without SBS
Radial Grafted.

The higher r2 using G*/sind at 0.125 rad/s
compared to G*/sind at 10.0 rad/s suggests that,
according to cumulative permanent shear strain, a low DSR frequency might provide
a better grading system. If the frequency is changed, then the criterion,
which is currently 2200 Pa after RTFO, must also be changed. Figure 11 shows
the relationship between cumulative permanent shear strain and temperature if
the frequency is changed to 0.125 rad/s, but the criterion is not changed.
The temperatures are very low and the correlation is poor. The lack of a known
correlation between cumulative permanent shear strain and pavement rutting makes
it difficult to choose a criterion. A preliminary recommendation based on the
ranking in table 6 is to use a maximum allowable shear strain of around 30 000 mm/m to 40 000 mm/m. Figure
7 shows that a shear strain of 30 000 mm/m (log 30
000 = 4.477) provides a criterion of around 60 Pa (log 60 1.8), although the
scatter in figure 7 shows that this will not provide a perfect grading system.
Furthermore, the relationship based on 0.125 rad/s in figure 7 is not better
than the relationship based on high-temperature PG in figure 10.

Table 10 and figure 12 provide the cumulative permanent shear strains for
the asphalt mixtures at 70°C and 5,000 cycles vs. the cumulative permanent shear
strains for the asphalt binders from repeated creep at 70°C and 100 cycles.
The correlation is poor, having an r2 of 0.58. If the data for
the PG 64-28 materials are removed, the r2 drops to 0.21. Based on
the mixture test results, the cumulative permanent shear strains for the Elvaloy
and EVA Grafted asphalt binders are high, while they are low for the PG 70-22
asphalt binder. The relationship should start at the zero-zero origin, but it
does not. Therefore, the relationship must be curvilinear. Figure 13 provides
a log-log relationship. This did not improve the correlation. The r2 of 0.38 is poor. The repeated creep is a new asphalt binder test and it is not
known if the protocols are the optimal protocols.

F. French PRT

The rut depths from the French PRT at 70°C are given in tables 11 and 12.(1) Table 12 shows that the mixture with SBS Radial Grafted had a high coefficient
of variation. Tests on the mixtures with EVA and SBS Radial Grafted were repeated.
The range in the replicate rut depths for these mixtures is relatively large
compared to the range in average rut depth for all modified asphalt binders.
The statistical ranking in table 11 shows that the rut depths for all mixtures,
except for the mixture with PG 64-28, had rut depths that were not different
at a 5-percent level of significance.

Figures 14 and 15 show the relationships between rut depth and G*/sind using
DSR frequencies of 0.9 and 10.0 rad/s, respectively. The G*/sind 's for EVA
may be low. If so, G*/sind underpredicted the relative rutting resistance provided
by EVA. Without EVA, frequencies of 0.9 and 10.0 rad/s provided r2's
of 0.83 and 0.91, respectively, compared to 0.54 and 0.56 with EVA. Without
both EVA and PG 64-28, frequencies of 0.9 and 10.0 rad/s provided r2's
of 0.69 and 0.67, respectively. The data point for PG 64-28 increases the
upward curvature of the relationship, while the data point for EVA tends to
flatten the relationship. If the 9.9-percent rut depth for SBS Radial Grafted
in table 12 were to be eliminated, G*/sind would also underpredict the relative
rutting resistance of this asphalt binder.

Figure 16 provides the correlation with high-temperature PG. The PG of EVA
agrees with mixture performance. This means that the G*/sind for EVA is not
increasing as rapidly as it should at temperatures immediately below its high-temperature
PG. EVA was found to have the lowest slope (G*/sind divided by temperature) around
the grading temperatures. The r2 of 0.89 drops to 0.57 without the
data for PG 64-28. Even so, no data point is more than 1.5°C away
from the regression line.

Unlike cumulative permanent shear strain, low and high DSR frequencies provided
approximately the same degree of correlation with rut depth, even though a cursory
review of the G*/sind 's in table 11 showed that the two frequencies did not provide
identical rankings for the asphalt binders. To examine this in more detail,
the G*/sind 's were linearly regressed. The r2 of 0.81 in figure 17
shows that the relationship was good. Without Elvaloy, the r2 is
0.97. Based on the rut depths in table 11, the G*/sind of 4110 Pa for Elvaloy
at 10 rad/s is low relative to the other asphalt binders. It should have the
highest G*/sind .

Table 9. Coefficients of determination
between RSCH and DSR properties.

Figure 18 provides the correlation with the cumulative permanent shear strains
from the asphalt binder repeated creep test. The r2 of 0.83 drops
to 0.19 without the data for PG 64-28. The narrow range in rut depth provided
by the French PRT makes it difficult to make a firm conclusion.

Figure 19 provides the relationship between the cumulative permanent shear
strain from RSCH at 70°C and the French PRT rut depth at 70°C . If the
data for the mixture with the PG 64-28 asphalt binder are excluded, the
remaining data indicate that the French PRT provided a narrower range in
performance compared to RSCH. The statistical rankings in tables 6 and 11 support
this finding. Table 6 shows that shear strain provided five statistical groups
(A through E), while table 11 shows that only the mixture with the PG 64-28
asphalt binder had a significantly different resistance to rutting according
to the French PRT. In a previous FHWA study, both tests agreed with full-scale
pavement rutting tests, although only five asphalt binders were evaluated and
only two of these binders were polymer-modified asphalt binders.(5)

The mixtures with EVA, EVA Grafted, and SBS Linear at an asphalt binder content
of 4.85 percent were tested using RSCH at 70°C to determine if the reduction
in asphalt binder content contributed to the differences between RSCH and the
French PRT. Table 13 shows that the cumulative permanent shear strains
for EVA Grafted and SBS Linear at an asphalt binder content of 4.85 percent
were not repeatable, so a conclusion could not be made.

G. Conclusions

The cumulative permanent shear strains from RSCH at 70°C were correlated to
the G*/sind 's of the asphalt binders at 70°C and three DSR frequencies:
10.0, 2.0, and 0.125 rad/s. The best correlation was provided by a frequency
of 0.125 rad/s. At 0.125 rad/s, G*/sind underpredicted the relative
rutting resistance provided by EVA and SBS Radial Grafted. G*/sind at the standard
frequency of 10.0 rad/s underpredicted the rutting resistances provided by EVA
and Elvaloy, and overpredicted the relative rutting resistance provided by the
unmodified PG 70-22 asphalt binder. High-temperature PG underpredicted
the relative rutting resistance provided by SBS Radial Grafted.

Based on the French PRT at 70°C , G*/sind underpredicted the relative rutting
resistance provided by EVA at both high and low DSR frequencies. However, the
high-temperature PG of EVA agreed with mixture performance. This means that
the G*/sind for this asphalt binder did not increase as rapidly as it should
have at temperatures immediately below its high-temperature PG of 75°C . The
correlation between high-temperature PG and the French PRT provided no obvious
outliers.

Grafting did not improve the rutting resistance of EVA. Grafting and geometry
had no effect on the rutting resistances of the SBS-modified asphalt binders
at 50°C . The effect at 70°C was marginal.

H. Recommendations

The French PRT indicated that the current Superpave binder specification
is valid. The G*/sind for one asphalt binder, EVA,
was low at 70°C , but its high-temperature PG agreed with mixture rutting
performance. No changes to the specification are recommended based on the
French PRT results.

The cumulative permanent shear strains from RSCH suggested that a low DSR
frequency, such as 0.125 rad/s, might provided a better grading system than
10.0 rad/s. However, it is not known whether this finding applies to
pavements or if it is related to the accelerated nature of the RSCH test.
This requires full-scale validation. Furthermore, G*/sind at 0.125 rad/s and 70°C did not clearly provide a better correlation
to RSCH than high-temperature PG.

Based on the coefficients of variation in tables 2, 7, and 13, a minimum
of five replicate specimens should be tested by RSCH per mixture.

Additional research is needed to evaluate various protocols for the asphalt
binder repeated creep test.

For similar studies involving fewer asphalt binders, it is imperative that
the high-temperature PG's of the asphalt binders be as close to each other
as possible. If the performances of the asphalt binders at one particular
temperature are of interest, then the most important property of the asphalt
binders, such as their G*/sind 's, must be as close
to each other as possible at this temperature. In this study, the high-temperature
PG's of the polymer-modified asphalt binders varied from 71°C to 77°C . The
G*/sind 's of the asphalt binders at the test temperature
of 70°C also varied significantly. Regression and ranking statistical analyses
were used to find which asphalt binders had properties that did not agree
with the properties of the other asphalt binders based on mixture rutting
resistance. For studies involving fewer asphalt binders, these analyses may
not provide a valid conclusion because of insufficient data points. Therefore,
the deviation in the high-temperature properties of the asphalt binders must
be so small that they should not affect mixture rutting resistance. An alternative
approach is to adjust the mixture test temperature according to the asphalt
binder property so that all of the asphalt binders should provide the same
resistance to rutting.

NCHRP Project 90-07, "Understanding the Performance of Modified Asphalt
Binders in Mixtures," Work Plan, Study in Progress, National Cooperative Highway
Research Program (NCHRP), Transportation Research Board, National Research
Council, Washington, D.C., 2001.

AASHTO TP5, "Method for Determining the Rheological Properties of Asphalt
Binder Using a Dynamic Shear Rheometer," AASHTO Provisional Standards,
American Association of State Highway and Transportation Officials, Washington,
D.C., April 2000 Edition.